is .blend file size proportional to the unit settings?
It comes down to the information needed to describe the scene.

When you have modifiers, you only store the input mesh and the settings of modifiers. So in the case of subdivision modifier (for example) you save only the low-polygon mesh and a number indicating how many subdivisions there are. When you open the file, Blender generates all the subdivided vertices and displays them - they are kept in system RAM when the file is open, but not saved when you save the file.

When you apply the modifier(s) all that generated geometry becomes real - you can change each vertex individually. The information how to generate all that is lost because the modifier is gone. So when you save the file, Blender needs to save all the geometry.



Unit settings have no effect for file size, you are saving the same amount of values. The values themselves are just different (instead of saving the number 1.5 you save 3.81 - inch vs cm). Blender stores all these numbers as 32bit floats so no matter the value they occupy the same space in memory: 32bits = 4 bytes for each.